STRUCTURE OP WOODY STEMS 197 



of cambium. The stems of Dicotyledons and Gymnosperms, 

 smce they mcrease in diameter by the addition of new layers 

 of xylem or wood on the outside of that previously formed, 

 are called exogenous stems. The stems of Monocotyledons are 

 called endogenous — a, term adopted when botanists had the erro- 

 neous notion that monocotyledonous stems grow by the addition 

 of new tissues on the inside of the older ones. 



Structure of Woody Stems 



Woody stems, characteristic of the shrubs and trees of 

 Dicotyledons and Gymnosperms, are fundamentally the same 

 in structure as herbaceous dicotyledonous stems, for the 



Fig. 176. — A drawing, partially diagrammatic, of the half of a cross section 

 of an Apple twig before developing the features typical of woody stems. 

 a, epidermis and outer part of cortex; 6, collenchyma; c, inner part of cortex; 

 d, bast fibers; e, conductive part of phloem; /, cambium; I , xylem; i, pith. 



circular arrangement of vascular bundles and presence of 

 cambium are likewise their distinctive structural features. 

 They, too, are exogenous. Their herbaceous tips, being similar 

 in structure to the herbaceous dicotyledonous stems just 

 described, need no special attention. (Fig. 176.) Aside from 



